The field of the invention relates to fuel vapor recovery systems coupled to internal combustion engines. In one particular aspect, the invention relates to air/fuel ratio control for engines equipped with fuel vapor recovery systems.
Fuel vapor recovery systems are commonly employed on modern motor vehicles to reduce atmospheric emissions of hydrocarbons. Typically, a storage canister containing activated charcoal is coupled to the fuel tank for adsorbing hydrocarbons which would otherwise be emitted into the atmosphere. Such storage canisters may also be utilized to capture hydrocarbons when filing the fuel tank. To cleanse the canisters, ambient air is occasionally purged through the canister for absorbing stored hydrocarbons and inducting the purged hydrocarbon vapors into the engine. In addition, fuel vapors are inducted directly from the fuel system into the engine. The rate of vapor flow, from the both fuel system and canister, is typically controlled by pulse width modulating an electronically actuated solenoid valve.
Fuel vapor recovery systems add complications to air/fuel ratio feedback control systems. Conventional air/fuel ratio control systems regulate the induction of fuel in linear proportion to a measurement of inducted airflow for achieving a desired air/fuel ratio. Feedback control is then utilized to trim the inducted fuel charge in response to an exhaust gas oxygen sensor for maintaining the desired air/fuel ratio. When fuel vapor recovery systems are employed in vehicles having air/fuel ratio feedback control, the induction of rich fuel vapors may occasionally exceed the range of authority of the air/fuel feedback control system. Further, when vapor purge is initiated, there may be a transient in air/fuel ratio during the response time of the feedback control system.
U.S. Pat. No. 4,715,340 issued to Cook et al addresses the above problems. More specifically, the rate of vapor flow is controlled to be proportional to a calculation of inducted airflow (or, similarly, desired fuel charge calculation) such that the overall inducted mixture of air, fuel, and fuel vapor remains within the feedback system's range of authority. Air/fuel ratio transients which would otherwise occur during the onset of vapor induction are also reduced by maintaining vapor flow proportional to inducted airflow. This is accomplished by actuating the solenoid valve of the vapor recovery system with an electrical signal having a pulse width proportional to a measurement of inducted airflow.
The inventor herein has recognized at least one disadvantage of the above and similar approaches. More specifically, vapor flow through the solenoid valve is linearly proportional to the pulse width of the actuating signal only when the pressure differential across the valve is above a critical value correlated with sonic flow. Below this value, vapor flow is also a function of manifold pressure. Accordingly, vapor flow is not always linearly proportional to airflow, and accurate air/fuel ratio feedback control will not be achieved. This disadvantage becomes more pronounced with engines having low (or even positive) manifold pressures during portions of their operating cycles such as, for example, multiple intake valves per cylinder engines, supercharged engines, and turbocharged engines.